1. Field of the Invention
The present invention relates to directional sliding pendulum seismic isolation systems and articulated sliding assembly therefor, and more particularly, to directional sliding pendulum seismic isolation systems and articulated sliding assemblies therefore, that can reduce seismic load applied to structures, such as bridges or general buildings, through directional pendulum motion and frictional sliding.
2. Description of the Related Art
Recently, multi-span continuous bridges are widely used. In general, such a multi-span continuous bridge is designed to have a single fixed point in the longitudinal direction of the bridge. FIG. 1a shows an example of the conventional multi-span continuous bridge. In the conventional 4-span continuous bridge, a fixed support 102 is installed on a fixed support pier 103, which is located in the middle of the 4-span continuous bridge, to restrict the longitudinal movement of the superstructure 101 of the bridge. Movable supports 107 are installed on movable support piers 104, 105 and 106 to permit free longitudinal movement of the superstructure 101 of the bridge. FIG. 1b is a schematic view illustrating the deformation of the 4-span continuous bridge of FIG. 1a when a seismic load is imparted thereto. Referring to FIG. 1b, the seismic load is applied to the superstructure 101 of the bridge in the arrow direction xe2x80x9cbxe2x80x9d by an earthquake ground motion expressed in the arrow direction xe2x80x9cUgxe2x80x9d. The superstructure 101 of the bridge moves in the longitudinal direction of the bridge due to the seismic load. If the frictional force is negligible at the movable supports, the seismic load imparted to the superstructure 101 of the bridge would be transmitted solely to the fixed support pier 102 through the fixed support 103. The fixed support pier 102 provided with the fixed support 103 would withstand the whole seismic load transmitted from the superstructure 101 of the bridge, and finally be forced to deform as shown FIG. 1b. If an excessive seismic load is applied to the fixed support pier 102, the bridge itself as well as the fixed support 103 of the fixed support pier 102 will be seriously damaged, consequently resulting in possible failure of the fixed support pier 102.
In traditional earthquake resistant design of bridges and general structures, the structural members, components and systems are required to have adequate amount strength and ductility in the event of strong earthquakes. However, the structures designed according to this strength design principle tend to experience severe damage or excessive deformation in the event of very strong earthquake even though they may not collapse. Therefore alternative methods have been developed that can protect structures from earthquakes within predetermined deformation limit. One of the most widely used protection methods is seismic isolation system. Because it has been proved to be very effective in the reduction of seismic load in recent earthquakes, the use of seismic isolation systems is on an increasing trend.
The basic principle of the seismic isolation system will be explained in connection with the earthquake actions. However, the seismic isolation systems according to the present invention are not restricted to the earthquake motion, and can be applied also to various kinds of dynamic loads applied to the structures.
If a structure 201 is fixed to the ground 202 as shown in FIG. 2a, it can be modeled as a single degree of freedom system as shown in FIG. 2b. The response of the structure to the earthquake action, such as base shear force and relative displacement can be estimated using response spectra.
FIGS. 2c and 2d show graphs of acceleration response spectra and graphs of displacement response spectra respectively as examples. The drawings show response spectra for two values of damping ratio. In the graph of FIG. 2c, the vertical axis indicates the spectral acceleration and the horizontal axis indicates the period. In the graph of FIG. 2d, the vertical axis indicates the spectral displacement and the horizontal axis indicates the period. The base shear force acting between the structure and the ground by the horizontal ground motion can be estimated from the acceleration response spectrum shown in FIG. 2c. That is, if the natural period and the damping ratio ("xgr"1 or "xgr"2) of the single degree of freedom are given, the spectral acceleration is read from the curves shown in FIG. 2c. If the obtained spectral acceleration value is multiplied by the mass of the structure, the base shear force is approximately found.
The relative displacement between the superstructure and the ground can be estimated from the displacement response spectrum shown in FIG. 2d. If the natural period of the single degree of freedom and the damping ratio are given, the spectral displacement is read from the curves shown in FIG. 2d. The obtained spectral displacement shows the relative displacement of the ground of the single degree of freedom.
As can be seen from the graph shown in FIG. 2c, generally, if the period becomes longer, the spectral acceleration is reduced. Moreover, in the same period, if the damping ratio becomes larger, the value of the spectral acceleration is reduced.
In the case of the spectral displacement, as can be seen from the graph shown in FIG. 2d, if the period becomes longer, the relative displacement is increased. Furthermore, in the same period, if the damping ratio becomes larger, the value of the spectral displacement is reduced.
In conclusion, if the period is longer and the damping ratio is higher, the spectral acceleration is reduced, and thereby the seismic force, i.e., floor shear force, becomes small. The seismic isolation systems adopt the above mechanical principle. For example, the seismic isolation system such as a high damping lead rubber bearing has mechanical properties that the horizontal stiffness is very small but the damping capacity is high.
As shown in FIG. 3a, if a seismic isolation system 203 is installed between the base frame and a ground 202, the natural period of the whole structural system becomes even longer, and also the damping ratio increases. Like this, if the natural period T becomes longer period Te or the damping ratio "xgr" is increased to a ration "xgr"e, then the seismic force can be reduced significantly, as can be seen from the graph shown in FIG. 3b. 
However, as shown in FIG. 3c, if the natural period becomes longer, the relative displacement increases. To restrict the increase of the relative displacement, dampers can be installed in addition to the conventional seismic isolation system having low damping capacity. One of the seismic isolation systems having high damping capacity and the long natural period, which do not require the additional dampers, is a sliding pendulum seismic isolation system. However, the sliding pendulum seismic isolation system used presently has a structure that a slider moves on a dish having a concave surface, and therefore if the seismic isolating period becomes longer, the diameter of the dish becomes even larger. In the case of bridges, generally, an area to install a seismic isolator on a pier or an abutment is extremely restricted. Therefore, a long span bridge requiring the seismic isolating period of a long-term has a difficulty in using the conventional sliding pendulum seismic isolation system of the dish type.
It is, therefore, an object of the present invention to provide a sliding pendulum seismic isolation system having a new configuration, which can be easily installed without limitations in an installation area.
It is another object of the present invention to provide a sliding pendulum seismic isolation system, which does not use dampers additionally employed in a conventional seismic isolation system that has low damping capacity.
It is a further object of the present invention to provide a sliding pendulum seismic isolation system, which moves in predetermined directions and yet effectively induces seismic isolation effects in all horizontal directions for the earthquake motion that is applied in arbitrary direction.
It is a still further object of the present invention to provide a sliding assembly, which has newly structured sliders, used in a directional sliding pendulum seismic isolation system. Even though the sliding assembly is located at any position, the surfaces of upper and lower sliders in contact with a friction channel of the sliding pendulum seismic isolation system are kept uniform, and thus the compressive force is always transferred to the friction channel through the center of the sliders.
To achieve the above objects, the present invention provides a directional sliding pendulum seismic isolation system, which reduces earthquake effects on the structures using sliding pendulum motion in selected directions.
The present invention provides bi-directional sliding pendulum seismic isolation systems for reducing seismic force acting on a structure by sliding pendulum movements, each system comprising a lower sliding plate forming a sliding path in a first direction; an upper sliding plate forming a sliding path in a second direction; and a sliding assembly for reducing the seismic force of the structure by performing a pendulum motion by sliding along the lower and upper sliding plates.
In the present invention, the lower and the upper sliding plates have sliding channels for sliding of the sliding assembly respectively, and the sliding assembly includes a main body, lower sliders sliding along the lower sliding channel, and upper sliders sliding along the upper sliding channel.
According to the embodiment of the present invention, the lower and the upper sliding plates have sliding channels for sliding of the sliding assembly, and the sliding assembly includes an upper main body on which an upper slider is mounted on an upper surface thereof, a lower main body on which a lower slider is mounted on a lower surface thereof, and elastic or elasto-plastic objects inserted between the lower and upper main bodies. In one application, the upper main body and lower main body of the sliding assembly can rotate freely around vertical axis
Further, in another embodiment of the present invention, the lower and the upper sliding plates have at least a pair of sliding channels for sliding of the sliding assembly, wherein the sliding assembly has a ratio of a predetermined width/height not to be overturned when the sliding assembly performs the pendulum motion, and wherein radius of curvature of an arc section of the upper sliding channel has a value smaller than radius of curvature of the first directional pendulum motion to prevent the upper slider from escaping from the upper sliding channel while the sliding assembly performs the pendulum motion in the lower sliding channel, and radius of curvature of an arc section of the lower sliding channel has a value smaller than radius of curvature of the second directional pendulum motion to prevent the lower slider from escaping from the lower sliding channel while the sliding assembly performs the pendulum motion in the upper sliding channel.
In the above embodiment, preferably, the elastic or elasto-plastic objects of the upper and lower separable sliding assembly are spheres having a predetermined elasticity and damping capacity, and the lower and the upper main bodies have hemispherical holes for mounting the spherical elastic or elasto-plastic objects respectively.
Further, in the above embodiment, preferably, the elastic or elasto-plastic objects of the upper and lower separable sliding assembly are spheres having a predetermined elasticity and damping capacity, and the lower and the upper main bodies have a hemispherical central hole for mounting the spherical elastic or elasto-plastic objects and a contour hole around the central hole respectively.
Further, in another embodiment, the lower and the upper main bodies have a hemispherical central hole and a contour hole around the central hole respectively, the spherical elastic or elasto-plastic object having a predetermined elasticity and damping capacity is mounted in the central hole, and annular elastic or elasto-plastic objects having a predetermined elasticity and damping capacity are mounted in the contour hole.
In another embodiment, the elastic or elasto-plastic object of the upper and lower separable sliding assembly is a disc type having a predetermined elasticity and damping capacity, and the lower and the upper main bodies have a hole for mounting the disc type elastic or elasto-plastic object respectively.
In the present invention, the sliding channels may be formed in multiple, and an escape preventing sill may be provided between the sliding channels to prevent the sliders of the sliding assembly from escaping from the sliding channels.
Further, the present invention provides uni-directional sliding pendulum seismic isolation systems for reducing seismic force of a structure by earthquake motion of one direction, each system comprising a sliding plate having a sliding channel forming a sliding path in one direction; and a sliding assembly for reducing the seismic force of the structure by performing pendulum motion by sliding along the sliding channel.
The present uni-directional sliding pendulum seismic isolation systems may be installed in multi-level to induce seismic isolation effects in all horizontal directions by performing pendulum motion in two directions horizontally.
Further, the present invention provides a sliding assembly used in a bi-directional sliding pendulum seismic isolation system, the sliding assembly comprising: a main body; a lower slider provided at a lower portion of the main body, the lower slider sliding along a lower sliding channel of a lower sliding plate of the bi-directional sliding pendulum seismic isolation system; and an upper slider provided at an upper portion of the main body, the upper slider sliding along an upper sliding channel of an upper sliding plate of the bi-directional sliding pendulum seismic isolation system.
In the embodiment of the sliding assembly, the lower and upper sliders includes a slider support; and a slider core mounted at an end of the slider support to freely rotate with respect to the slider support, the slider core being in frictional contact with the sliding channels in such a manner that the area contacting the sliding channels remains unchanged even though the sliding assembly is located in an arbitrary position in the sliding channels.
Further, in the embodiment of the sliding assembly, the slider core has an upper surface of a shape corresponding to radius of curvature of the sliding channels and a lower surface of a semicircular plate type having a predetermined thickness and radius of curvature, and rotates with respect to the slider support when the lower surface is mounted in the slider support.
In another embodiment of the sliding assembly, the slider core has an upper surface of a shape corresponding to radius of curvature of the sliding channels and a lower surface of a round shape having a predetermined radius of curvature, and rotates with respect to the slider support when the slider core is inserted in the slider support.
In another embodiment of the sliding assembly, the slider includes a slider support having a disc type supporting part of a predetermined thickness and radius of curvature of a convex form at an end; and a slider core having an upper surface of a shape corresponding to the radius of curvature of the sliding channels and a concave part corresponding to the disc type supporting part, the slider core being mounted on the slider support in such a manner that the disc type supporting part is inserted into the concave part. The slider core can rotate freely with respect to the slider support.
In another embodiment of the sliding assembly, the slider includes a slider support having a spherical supporting part of a predetermined radius of curvature, which is in the form of a convex at an end; and a slider core having an upper surface of a shape corresponding to the radius of curvature of the sliding channels and a concave part corresponding to the spherical supporting part, the slider core being mounted on the slider support in such a manner that the spherical supporting part is inserted into the concave part, the slider core freely rotating with respect to the slider support.
Preferably, in the sliding assembly, friction-reducing materials are coated on the surface of the slider core to reduce a friction between the slider core and the sliding channel and a friction between the slider core and the slider support.
The present invention also provides a sliding assembly used in a unidirectional sliding pendulum seismic isolation system, the sliding assembly comprising a main body; and a slider formed at an upper portion of the main body, the slider sliding along the sliding channel of the sliding plate of the unidirectional sliding pendulum seismic isolation system, wherein the slider includes a perpendicular slider support and a slider core mounted at an end of the slider support and being in frictional contact with the sliding channel, and wherein the slider core is mounted to rotate with respect to the slider support and maintains contact area with the sliding channels even though the sliding assembly is located in an arbitrary position in the sliding channels.